Several neurochemical phenomena have been investigated to determine the extent to which they might correlate with sex specific differentiation of the rat brain. Monoamine oxidase (MAO) levels were found to be sex specific in a region within the hypothalamus of the 12 day-old rat. Levels of MAO types A and B could be influenced by androgen levels in the neonate. Levels of norepinephrine, dopamine, and serotonin were sex specific in the hypothalamus, midbrain, and/or corpus striatum. Hypothalamic norepinephrine and/or midbrain serotonin in the 12 day-old rat were appropriately altered by manipulations which could be expected to influence sexual differentiation of the brain.